To record/reproduce information with respect to a recording medium typified by an optical disk, the emission power of a laser has to be optimized for an information recording surface of the medium (described as a disk in the following). In general, the properties of a semiconductor laser vary significantly depending on ambient temperature changes or degradation. Therefore, the semiconductor laser requires a control means that enables the output of a power suitable for recording/reproducing information with respect to the disk so as to accommodate variations in the properties.
The properties of the semiconductor laser will be described briefly below. The following description gives an example that uses the semiconductor laser as a laser.
FIG. 7 shows the I-L characteristics (injection current-light intensity characteristics) of the semiconductor laser at temperatures T1 and T2. The oscillation of the laser starts when the laser is driven by a current larger than a threshold current Ith. In the oscillation region, the optical output per unit drive current increases in proportion to the quantum efficiency η.
In FIG. 7, the graphs show the threshold current Ith0 and the quantum efficiency η0 at T1 and the threshold current Ith1 and the quantum efficiency η1 at T2. The threshold current Ith and the quantum efficiency η change with the ambient temperature, and their changes differ from laser to laser. In some lasers, the threshold current Ith and the quantum efficiency η at T2 may be two or more times greater than those at T1 depending on temperature changes, as indicated by the graphs at T1 and T2 in FIG. 7. Thus, even if the drive current of the laser is the same, the power of the optical output emitted from the laser changes significantly with the ambient environment. Therefore, laser power control that modifies the drive current in accordance with the ambient environment generally is performed so as to adjust the power of the optical output to the intended power (see, e.g., Patent Document 1).
FIG. 8 is a block diagram showing an example of an optical disk apparatus using a conventional laser power control method. In FIG. 8, a pickup 2 provided with a laser 1 includes a front photodetector 4 that receives a light beam emitted from the laser 1 and converts the power of the light beam into an electric signal, and a driver 60.
A disk 3 is rotated by a motor 5 at a predetermined rotational speed, and data is recorded on the disk 3 in accordance with the power of the light beam output from the pickup 2. For reproduction, a photodetector 6 receives the light beam reflected by the disk 3 and converts the power of the light beam into an electric signal. Each track of the disk 3 is divided into sectors that are used as the unit of data recording, and an address area where an address for identifying the sector is recorded is placed at the beginning of each sector. The appropriate power for recording data on the disk 3 requires a plurality of levels to form a recording mark, and each of the levels differs from one track to another.
The front photodetector 4 samples and holds the power of the received light beam, converts the power into an electric signal, and outputs the electric signal to an A/D converter 7. The A/D converter 7 performs an analog-to-digital conversion of the signal input from the front photodetector 4 and then outputs the signal to a controller 8. Upon receiving the output of the front photodetector 4 via the A/D converter 7, the controller 8 determines and outputs drive values required for the laser 1 to output a plurality of recording power levels at which data is recorded on the disk 3. In this case, however, the appropriate drive values of the laser 1 differ between land track recording and groove track recording on the disk 3, as will be described later. Therefore, the controller 8 outputs the drive values corresponding to the land track recording and the groove track recording, respectively. Each of the drive values is switched by a switch 12 and supplied to the driver 60. A land/groove controller 20 outputs a land/groove signal (LGS) to the switch 12. The LGS is a control signal for switching the appropriate recording levels that differ from one track to another of the disk 3. The driver 60 drives the laser 1 in accordance with the drive values.
A DVD-RAM is known as a disk with a structure in which the appropriate power differs from one track to another, such as the disk 3. In a disk typified by the DVD-RAM, a pulse train is used to form a recording mark, and the power of the optical output emitted from a laser has a plurality of levels corresponding to the pulse train. Here, an example of three power levels will be described. For the DVD-RAM, the control powers are defined as a peak power, a bias power 1, and a bias power 2. In the following explanation, the three respective control powers are abbreviated as PPK, PB1, and PB2. In the disk 3, the tracks on which data can be recorded are provided in guide grooves and on lands between the guide grooves. The guide grooves are referred to as groove tracks, and the lands between the guide grooves are referred to as land tracks. Each of the levels PPK, PB1, and PB2 differs between the groove track and the land track.
In this context, the different power levels for the groove and land tracks should be described along with the levels of the control powers defined as PPK, PB1, and PB2. For ease of distinction, the levels PPK, PB1, and PB2 are referred to as the control powers of a first definition group. Moreover, the different power levels for the groove and land tracks are categorized by the type of tracks and referred to as the levels of a second definition group. The levels of the second definition group for the land track are represented by Land (PPKL, PB1L, PB2L), and the levels of the second definition group for the groove track are represented by Groove (PPKG, PB1G, PB2G).
FIG. 9 shows an example of the three power levels emitted from the laser to form a recording mark. FIG. 9(a) shows a timing waveform MRS that indicates a period during which a recording mark is formed with two levels of high (H) and low (L), and the recording mark is formed on the medium during the H level period. FIG. 9(b) shows the land/groove signal (LGS) in FIG. 8. The LGS at the L level indicates a land track section, and the LGS at the H level indicates a groove track section. FIG. 9(c) is a conceptual diagram showing power levels RPL of the optical output emitted from the laser during the H level period in FIG. 9(a). With 0 mV indicated by the broken line as a reference, three power levels PPKL=10 mW, PB1L=5 mW, and PB2L=1 mW are switched at high speed. In the case of 16× speed recording, the switching between PPK and PB1 occurs about every 3 ns, which is substantially in the vicinity of the time interval between the rise and fall of the pulse of the semiconductor laser.
In order to properly receive the three power levels of the optical output that are switched at high speed, a high-bandwidth light receiving element needs to be used as the front photodetector 4 in FIG. 8. However, the high-bandwidth light receiving element is very expensive, so that the cost of the apparatus is increased. Therefore, it is better to use a low-bandwidth light receiving element to suppress an increase in cost. In such a case, however, the low-bandwidth light receiving element cannot properly detect the three power levels of the optical output. As shown in FIG. 9(c), at the time of transition from the land track to the groove track, the levels of the second definition group have to be changed from Land (10 mW, 5 mW, 1 mW) to Groove (14 mW, 7 mW, 1 mW). The speed of switching these drive levels depends on the ability of the driver 60 in FIG. 8. Like the light receiving element described above, a drive circuit that can switch at a high speed of several 100 ns is very expensive. Thus, a drive circuit with a switching speed of about several μs generally is used.
FIG. 10(a) schematically shows a state in which the light beam moves along a track by the rotation of the disk. The track is divided equally into sectors, and an address area where an address for identifying the sector is recorded is placed at the beginning of each sector. Here, the address area is referred to particularly as a header area, and an area that is placed immediately after the header area is referred to as a gap area where no data to be reproduced is present and no data is recorded. Moreover, a data area for recording data follows the gap area. In FIG. 10, the vertical broken lines indicate synchronous timing with the light beam when it is located in each area of FIG. 10(a). FIG. 10(b) shows the land/groove signal (LGS) in FIG. 8. The LGS at the L level indicates that the light beam is located in the land track, and the LGS at the H level indicates that the light beam is located in the groove track.
FIG. 10(c) schematically shows levels PL of the emission power of the laser. When the three power levels required for recording are switched at high speed, as described above, while the light beam passes through the gap area, a high-speed photodetector needs to be used to monitor the output power directly with high precision. However, the use of the high-speed photodetector increases the cost of the apparatus. To avoid this, the power switching timing should be slow enough to monitor the output power. However, accurate recording cannot be performed if the power switching timing is changed during information recording. Thus, the controller 8 is operated by performing a test light emission at low speed while the light beam passes through the gap area other than the information recording.
In the following explanation, PPKT and PBT represent two power levels obtained by the front photodetector 4 in FIG. 8 as a result of the test light emission. The controller 8 can determine the quantum efficiency η and the threshold current Ith of the laser based on the following equations.η=(PPKT−PBT)/(IPKT−IBT)Ith=IBT−PBT/ηwhere IPKT represents a drive current to output PPKT and IBT represents a drive current to output PBT.
Substituting the resultant values of η and Ith in the following equations, it is possible to determine drive currents IPK, IB1, and IB2 that provide powers PPK, PB1, and PB2 required for recording information in accordance with the I-L characteristic variations due to the ambient temperature.IPK=1/η×PPK+Ith IB1=1/η×PB1+Ith IB2=1/η×PB2+Ith When the light beam reaches the data area, the power switching timing is changed to high speed such that information can be recorded, and the drive currents IPK, IB1, and IB2 calculated by the controller 8 are output to the driver 60. In this case, the drive currents are represented by Land (IPKL, IB1L, IB2L) when the light beam is located in the land track and by Groove (IPKG, IB1G, IB2G) when the light beam is located in the groove track.
For the land track, the drive currents can be determined by the following equations.IPKL=1/η×PPKL+Ith IB1L=1/η×PB1L+Ith IB2L=1/η×PB2L+Ith For the groove track, the drive currents can be determined by the following equations.IPKG=1/η×PPKG+Ith IB1G=1/η×PB1G+Ith IB2G=1/η×PB2G+Ith 
When the light beam is located at the leading end of the data area after having passed through the gap area in FIG. 10(a), the driver 60 switches between Land (IPKL, IB1L, IB2L) and Groove (IPKG, IB1G, IB2G) for driving within a predetermined time.
In the above explanation, as an example of the operation of the controller 8, the drive currents are determined by the operation using a processor such as a DSP or microcomputer. However, the method described in Patent Document 2 also can be applied.    Patent Document 1: JP H6(1994)-236576 A    Patent Document 2: JP H7(1995)-111783 A